kernel/locking/rwsem.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 /*
   3  * The owner field of the rw_semaphore structure will be set to
   4  * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
   5  * the owner field when it unlocks. A reader, on the other hand, will
   6  * not touch the owner field when it unlocks.
   7  *
   8  * In essence, the owner field now has the following 3 states:
   9  *  1) 0
  10  *     - lock is free or the owner hasn't set the field yet
  11  *  2) RWSEM_READER_OWNED
  12  *     - lock is currently or previously owned by readers (lock is free
  13  *       or not set by owner yet)
  14  *  3) Other non-zero value
  15  *     - a writer owns the lock
  16  */
  17 #define RWSEM_READER_OWNED      ((struct task_struct *)1UL)
  18
  19 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  20 /*
  21  * All writes to owner are protected by WRITE_ONCE() to make sure that
  22  * store tearing can't happen as optimistic spinners may read and use
  23  * the owner value concurrently without lock. Read from owner, however,
  24  * may not need READ_ONCE() as long as the pointer value is only used
  25  * for comparison and isn't being dereferenced.
  26  */
  27 static inline void rwsem_set_owner(struct rw_semaphore *sem)
  28 {
  29         WRITE_ONCE(sem->owner, current);
  30 }
  31
  32 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
  33 {
  34         WRITE_ONCE(sem->owner, NULL);
  35 }
  36
  37 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
  38 {
  39         /*
  40          * We check the owner value first to make sure that we will only
  41          * do a write to the rwsem cacheline when it is really necessary
  42          * to minimize cacheline contention.
  43          */
  44         if (sem->owner != RWSEM_READER_OWNED)
  45                 WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
  46 }
  47
  48 static inline bool rwsem_owner_is_writer(struct task_struct *owner)
  49 {
  50         return owner && owner != RWSEM_READER_OWNED;
  51 }
  52
  53 static inline bool rwsem_owner_is_reader(struct task_struct *owner)
  54 {
  55         return owner == RWSEM_READER_OWNED;
  56 }
  57 #else
  58 static inline void rwsem_set_owner(struct rw_semaphore *sem)
  59 {
  60 }
  61
  62 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
  63 {
  64 }
  65
  66 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
  67 {
  68 }
  69 #endif